Intermodulation distortion is a form of signal distortion caused by an unwanted amplitude modulation of signals that occurs due to passage through a non-linear channel. Such intermodulation can cause additional signals that are present at various combinations of sums and differences of the frequencies that constituted the original source signals. Such intermodulation can cause unwanted signal components to appear in other frequency bands and cause interference to other useful signals. In addition, this intermodulation can cause signal distortion throughout the original band and thus reduce the quality of information-carrying signal transmissions.
The presence of intermodulation distortion, particularly on the downlink path, can be a serious problem for communication network operators. This distortion, which may corrupt existing communication signals and/or occupy frequency bands that are allocated for other purposes, can cause the communication network to fail to achieve its design throughput capacity. One known cause of intermodulation distortion is due to cabling and/or connector malfunction or is caused by other passive components. Such intermodulation distortion is referred to herein as Passive Intermodulation Distortion (“PIM”). Unfortunately, intermodulation distortion is not easy to identify without active investigation, such as disconnecting components and subjecting them to scrutiny and/or injecting test signals via sophisticated diagnostics equipment. Such investigation and testing is costly and time-consuming and requires at least a portion of the communication network to be out of operation for a period of time. Furthermore, cables, connectors, and other passive circuit elements may deteriorate over time due to a variety of reasons including weather and the local operating environment. Thus, for example, cables and connections that check out fine at the time of installation or testing may deteriorate without notice until they cause a decrement in network operation.
Intermodulation distortion is often present in many operational communication networks and the network operator may not be aware until the problem becomes large enough that is causes major interference with data and/or voice carrying communication channels.
Accordingly, there is a need for identifying signals which may be the result of intermodulation distortion, determining intermodulation distortion from two or more radio frequency (“RF”) signals, and determining intermodulation distortion in a communication system which operates using known RF channels and a known communication signal type.